This invention relates to an electronic control system of an automatic transmission for internal combustion engines, and more particularly to a back-up device for the electronic control system.
In an electronic control system for electronically controlling the gear shifting operation of an automatic transmission in an internal combustion engine for automotive vehicles, when an electronic control unit of the system malfunctions to select a gear having too low a speed reduction ratio or a reduction ratio too large for a speed at which the vehicle is running, the engine undergoes overrevolution and can be damaged. Supposing for instance that when a vehicle equipped with an automatic transmission having four forward speed ratios, is running with the fourth speed ratio (TOP) established, its electronic control unit becomes defective so that the second speed gear or the first speed gear (LOW) is selected, the engine will undergo overrevolution and can be damaged owing to a large difference in gear ratio between the fourth speed gear and the second speed gear or the first speed gear (LOW). Particularly, since no one-way clutch which permits transmission of torque only from the engine to the driving wheels is provided between the second speed driven gear and the counter shaft, the engine is driven by the driving wheels when gear shifting takes place from the fourth speed gear to the second speed gear, and can be damaged by overrevolution even if the throttle pedal is returned from its stepped-on position. The possibility of damage to the engine due to overrevolution is high particularly when gear shifting takes place from the fourth speed ratio to the second speed ratio or the first speed ratio while the vehicle is running at a high speed. Similarly the engine can undergo overrevolution when the electronic control unit malfunctions to cause gear shifting from the fourth speed ratio to the third speed ratio while the vehicle is running with the fourth speed ratio established. However, since generally the third speed gear ratio is relatively small and suitable for high speed running, the possibility of overrevolution of the engine is very small.